The subject invention relates to a method and apparatus for recovering environmentally hazardous metals from flue dust or fly ash which are generally considered waste byproducts of primary pyrometallurgical processes. More particularly, it is a continuous feed shaft retort process for recover of non-ferrous metals from secondarily oxidized flue dusts recovered from primary pyrometallurgical metal producing processes. The invention is particularly adaptable for use with an inclined rotary reduction smelter or other primary metallurgical process from which energy is derived to supply the present process, but which energy would otherwise be wasted. The invention is also operable by using primary energy sources. The present invention is useful for recovering all of the known "volatile metals". These include, without limitation, zinc, lead, sodium, potassium, cadmium, arsenic, mercury, and barium.
The invented apparatus will accomplish certain results with energy savings not heretofore available to vertical retort processes for recovery of non-ferrous metals. Previous vertical retort processes such as invented by E. C. Handwerk et al, in U.S. Pat. 2,457,552, were specifically designed to smelt zinc from high grade (oxide or oxidized) primary zinc ores. The object of the Handwerk et al patent was to overcome certain technical and economic problems inherent with previous horizontal and vertical retort processes. Premature condensing of zinc vapors on the cold burden as the vapors rose through the unheated extension (top) of the vertical retort resulted in an excessive recirculating load of zinc within the retort and hindered the free movement of the burden downward through the retort; thus decreasing productivity and increasing the operating costs of the process.
To overcome the "refluxing" (which is the condensing of volatile metal on the cold burden) actions of the zinc vapor and prematurely condensed liquid metal on the burden in the top of the retort, the burden had to be preheated or precoked at temperatures between 850 and 900.degree. C. before being fed to the vertical retort. Such precoking also served to remove moisture and volatile matter from the carbon reductant prior to the burden entering the vertical retort and thereby reducing the evolution of process limiting H.sub.2 O, and CH.sub.4 gases inside the retort. Preheating, or precoking, actions thus improved the productivity and economics of the vertical retort process.
Precoking of the burden involves the use of an independent autogenous grate coker in which a blended burden of oxidized zinc ore, bituminous coal, anthracite coal and/or coke are burned under oxygen controlled atmospheric condition to prevent the temperature in the bed of the burden from exceeding the point at which premature reduction of the zinc oxide to zinc metal and vapor occurs. By precoking the vertical retort burden material, sufficient agglomeration strength is achieved to allow the material to withstand the rigors of passing down through the shaft of the retort without undergoing excessive disintegration which would cause loss of bed permeability in the shaft and inhibit the free passage of gases through the bed.
As the autogenous coking step is a prerequisite for the burden before it enters the Handwerk et al vertical retort process, certain environmental problems are inherent. It is known that in the case of smelting electric arc furnace (EAF) flue dusts, which contain oxides of non-ferrous metals, reduction and vaporization of the metals begin to occur in the same temperature range as is employed in the autogenous coker. While the gases generated in the autogenous coker pass through an afterburning step, waste gas scrubbing has not been thought to be necessary. Thus, reoxidized fumes of non-ferrous metals can be emitted with the stack gases from the coker, creating a potential environmental hazard in the area surrounding the autogenous coker.
Further, between the autogenous coking step and the Handwerk et al vertical retort step, the hot coked material must be protected from atmospheric air to prevent uncontrolled ignition and sintering of the burden. Such uncontrolled sintering would clinker the burden and release volatilized metal fumes to the atmosphere.
In Handwerk et al Pat. No. 2,457,552, a claim is made to introduce air, or a combustion supporting gas (which implies a possible mixture of air and oxygen), into the bottom of the retort for the purpose of igniting with residue carbon in the burden to generate additional heat and "displacement" gas inside the retort. Careful control is necessary to prevent sintering or clinkering of the burden in the shaft of the retort and to avoid generating excessive amounts of diluting gases, especially process limiting carbon dioxide. By burning the residue carbon in the spent burden exiting the heated section of the retort, additional heat and products-of-combustion gas (displacement gas) is generated. The hot displacement gas transfers the heat upward into the shaft burden, thus improving the energy efficiency of the process.
The New Jersey Zinc (Handwerk) vertical retort has straight vertical sides whereas the retort of the present invention has inclined sides with a larger cross-section at the bottom than at the top. The New Jersey Zinc vertical retort requires coke briquets and processes only high grade material and takes about twenty-four hours to treat the material. The invented apparatus utilizes only pellets which need only be dried and not pre-coked processes any type of metallized oxides, particularly low grade materials, and does so in a considerably shorter time, averaging only about four hours residence time of the material within the retort